Perfluorinated sulphonic acids are effective catalysts in the alkylation of a paraffinic feedstock with olefinic alkylation agents such as alkylation of isobutane with propene and butenes as described in U.S. Pat. Nos. 5,220,095, 5,245,100 and 5,498,820. The alkylation process typically produces various amounts of spent acid catalyst. Byproducts formed in the alkylation reaction deactivate the spent acid.
To improve process economy of the processes involving perfluorinated sulphonic acid catalysts it is highly desirable to recover spent acid catalyst and recycle the catalyst in its active form.
Several attempts have been made in the past to recover valuable amounts of spent perfluorinated sulphonic acid catalyst from alkylation residues. U.S. Pat. No. 6,340,414 thus discloses recovery of spent acid by heating and distillation. Recovery of a fluorinated sulphonic acid from a spent acid product by steps of subjecting the spent acid product to thermal treatment to obtain the acid in its active form and a residue and separating the active acid from the residue by stripping with a stripping agent and separating the stripping agent from the active acid by distillation is known from EP-A-1,044,963.
By the above known recovery processes still valuable amounts perfluorinated sulphonic acid is contained in a viscous residue being formed during recovery.
DK PA 2001 00809 discloses a method to treat such viscous residues for the further recovery of acid by adding an alkyl ammonium salt of the acid to be recovered and subsequently extraction of the acid.
Viscosity of the residues increases as the acid is extracted. In some cases the organic part of the residue even solidifies and encapsulates the acid. The method of DK PA 2001 00809 requires therefore vigorous agitation and heating of the residue during extraction of the acid.